This invention relates to the disposal of solid wastes, especially with the process of detoxification and comprehensive utilization of chromic residues.
A large amount of chromic residues is discharged during the production of chromates by calcination of chromic ores. Statistics show that to produce a ton of sodium dichromate will discharge 2.5-3 tons of poisonous chromic residues, which contain 3-7% of residual Cr2O3, 8-11% of Fe2O3, and 0.5-1.5% of water-soluble Cr+6, the latter being poisonous to both human beings and animals. Discarding of metal oxides contained therein is not only a waste of resources, but will also pollute the environment.
There have been a large number of reports at home and abroad, concerning the disposal of chromic residues, which methods can be mainly put into two categories. One is by reduction, i.e., to reduce hexavalent chromium in the residues to trivalent, and then dispose the reduced residues by deep bury, discarding into the sea, or stacking within solid cache. On the basis of a thermal reduction disclosed in a USSR Patent 975580, CN 85105628 suggested a process, by which chromic residues were first extracted with hydrochloric acid, and then reduced by a reductant added. CN.90103420 reported a process to reduce chromic residues under high temperature and vacuum conditions. Any of the reduction process, however, bears the problem of trivalent chromium going up back to hexavalent, owing especially to the existence of oxides of calcium, magnesium and other metals in the chromic residues. Another method is to utilize chromic residues comprehensively as an industrial raw material, so as to not only consume the residues, but also gain some economic benefits. As shown in the Journal of "Wujiyan Gongye" (Chinese), No. 2, 1987, and CN 85105628, chromic residues were used for the coloration of glass, for the production of colored cement, bricks and calcium/magnesium/phosphorus fertilizers, as well as for iron smelting as a subsidiary material. Nevertheless, this method can neither be put into an industrial-scale production due to limited market, nor be spread readily due to complicated technology and high cost.
Many reports have revealed a process developed recently, by which chromic residues were reduced at high temperatures with subsidiary materials added, to realize the detoxification of chromic residues, as well as a comprehensive utilization thereof. As was disclosed in CN 88104766, chromic residues, calcined as the main component, were reduced at high temperatures in a blast furnace, with potash spar and lime added as subsidiary components, to form detoxified slags, which were used for the production of cement, potash fertilizer and chromium-containing cast iron. However, the direct smelting of sodium-containing (7-10%) chromic residues will lead to the formation of furnace nodule(s) and affect the strength of the same, thus shortening the service of the furnace.